How do I create a class object in Lua-C API 5.2?

I'm wrapping a C function with Lua, using the Lua-C API for Lua 5.2:

#include <lua.h>
#include <lauxlib.h>
#include <stdlib.h>
#include <stdio.h>

int foo_gc();
int foo_index();
int foo_newindex();
int foo_dosomething();
int foo_new();

struct foo {
  int x;
};

static const luaL_Reg _meta[] = {
    {"__gc", foo_gc},
    {"__index", foo_index},
    {"__newindex", foo_newindex},
    { NULL, NULL }
};
static const luaL_Reg _methods[] = {
    {"new", foo_new},
    {"dosomething", foo_dosomething},
    { NULL, NULL }
};

int foo_gc(lua_State* L) {
  printf("## __gc\n");
  return 0;
}
int foo_newindex(lua_State* L) {
  printf("## __newindex\n");
  return 0;
}
int foo_index(lua_State* L) {
  printf("## __index\n");
  return 0;
}
int foo_dosomething(lua_State* L) {
  printf("## dosomething\n");
  return 0;
}
int foo_new(lua_State* L) {
  printf("## new\n");

  lua_newuserdata(L,sizeof(Foo));
  luaL_getmetatable(L, "Foo");
    lua_setmetatable(L, -2); 

  return 1;
}

void register_foo_class(lua_State* L) {
    luaL_newlib(L, _methods); 
  luaL_newmetatable(L, "Foo");
  luaL_setfuncs(L, _meta, 0);
  lua_setmetatable(L, -2);
  lua_setglobal(L, "Foo");
}

When I run this Lua:

local foo = Foo.new()
foo:dosomething()

...I see this output (with error):

## new
## __index
Failed to run script: script.lua:2: attempt to call method 'dosomething' (a nil value)

What am I doing wrong?

Solution 1:

Ok, got it working. I had to add __index and __metatable to Foo's new metatable, as shown below:

void register_foo_class(lua_State* L) {
  int lib_id, meta_id;

  /* newclass = {} */
  lua_createtable(L, 0, 0);
  lib_id = lua_gettop(L);

  /* metatable = {} */
  luaL_newmetatable(L, "Foo");
  meta_id = lua_gettop(L);
  luaL_setfuncs(L, _meta, 0);

  /* metatable.__index = _methods */
  luaL_newlib(L, _methods);
  lua_setfield(L, meta_id, "__index");  

  /* metatable.__metatable = _meta */
  luaL_newlib(L, _meta);
  lua_setfield(L, meta_id, "__metatable");

  /* class.__metatable = metatable */
  lua_setmetatable(L, lib_id);

  /* _G["Foo"] = newclass */
  lua_setglobal(L, "Foo");
}

Solution 2:

I tried replying to your solution but apparently I don't have the reputation to do so yet, so here goes a separate answer.

Your solution is pretty nice, but it does not allow for something that I'd like to do: Have both "array-like" access to an object and still have functions on it. Have a look at this Lua code:

Foo = {}

mt = {
__index = function(table, key)
  print("Accessing array index ", tostring(key), "\n")
  return 42
end
}
setmetatable(Foo, mt)

Foo.bar = function()
  return 43
end

print(tostring(Foo[13]), "\n")
print(tostring(Foo.bar()), "\n")

--[[
Output:
Accessing array index 13
42
43
]]--

Registering a class using your solution does not seem to allow for this, as the __index entry is overwritten. It might not make sense to have both array access and function access on a class, but for the sake of simplicity (offering one C function for registering both types of classes) I'd like to use the same code everywhere. Does anyone have an idea how this restriction could be circumvented, so that I can create a class from C which has both a function Foo.bar() but also Foo[13]?

How do I create a class object in Lua-C API 5.2?

Solution 1:

Solution 2:

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